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Old 28th March 2014, 12:42 PM   #11
JimW is offline JimW  United States
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If you are going to use a common filament winding (as your wiring diagram seems to indicate) you will not be able to measure the bias setting to each tube individually. You would have to have a separate winding for each tube. I have a PP 2a3 amp, cathode biased, and I feed each 2a3 with its own CT transformer, and connect the center tap of each to a cathode resistor. If you wish to use fixed bias and have the ability to measure each tube individually, I would suggest using a similar arrangement, a 10 ohm resister from center tap to ground of each tube would allow you to read each tubes bias separately. If you are going to use a common winding for both tubes, use its center tap to feed a 5 ohm resistor to ground, would allow you to read the bias current in ma for each tube in ma, but they would be biased as a pair ( ie a reading of 50mv across the 5 ohm would indicate each tube is running at 50 ma, assuming they are a matched pair). I personally do not care for schemes that require measuring the B+ side - too easy to make a mistake. If you have two bias pots installed, and want to have the ability to set each tube, you will have to have two separate filament supplies, each with either a center tap, or a derived center tap using a pair of resistors from each side of the filament to ground.
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Old 29th March 2014, 04:13 AM   #12
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Thanks, I now have a plan. I found a 2.5 volt 3 amp transformer made by Hammond. Angela sells it for $14.

Jim, To measure current I would need the voltage dropped across a resistor. So why a pair?
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Old 29th March 2014, 04:52 AM   #13
AJT is offline AJT  Philippines
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if using a 2.5 volt heater winding with ct. 1 resistor to the ct is enough.
if no center tap then 2 resistors on either side of the filament...

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Old 29th March 2014, 02:00 PM   #14
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Will the DC voltage dropped across one of the two resistors be the same as the other? I.E will I only have to measure across one? And the AC current from the filament. does that have to be taken into account when choosing the resistor? It would have to be something like a ten watt resistor correct?
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Old 30th March 2014, 02:55 PM   #15
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Hey All,

I suspect I am trying your patience with this so maybe I can get an answer if I tell you what I think will happen? With two resistors to ground in a faux center tap I should be able to measure the voltage drop across one resistor and be happy. Kirchoff tells me that current is the same throughout the circuit. So it will be the same across each resistor. When the AC filament voltage swings positive it will add to the DC current. When it swings negative it will subtract. And this will happen very fast so the end result will be no effect. Couple this with the fact that 2.5 volts is only .00666..% of the B+ of 300 volts. Leads me to believe I can ignore the AC in this application.

Last edited by mr2racer; 30th March 2014 at 02:58 PM.
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Old 30th March 2014, 07:11 PM   #16
waltube is offline waltube  Italy
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The finest solution is to have two separate ac wiring 2,5-0-2,5 vac one for each 2A3.
The center tap goes to ground by 1 ohm -2w resistor where you can check the current; this is the easiest way to do this ( with a test point on chassis)
The use of two filaments is not only the way to check the current of the tube but also to reduce the hum ( in 2A3 is low) because you can swap indipendently (on each tube) the wire to find the best results.

Bye

Wlater
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Old 30th March 2014, 08:22 PM   #17
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I may have a solution that will fit the amp as it is. If I can find, or have built, an isolation transformer with a single primary capable of 2.5 volts and 6 amps. And two secondaries capable of 2.5 volts at three amps each both with center taps I'd have it made.
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Old 31st March 2014, 06:54 PM   #18
JimW is offline JimW  United States
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Assuming ac heating, if you want to use a single transformer and secondary, get one rated for 2.5 volts at 5-6 amps, and if a centertap is available, use the resistance value of your choice to ground. If a ct is not available, then you are correct that you can use two resistors to make a faux centertap. I think hammond makes a transformer with dual 2.5 volt secondaries, which would allow you to have a single transformer and still have the ability to monitor each tube individually. If you use a single winding, you may find that hum levels can be influenced by which pins you choose to parallel, for example wiring pins 1 to 1 and four to four might show more hum than 1 to four and four to 1, depending on the tubes. ( I found this on a 300B pp amp I built a few years back, wired a switch in so I could reverse the "polarity" of one of the filaments for least hum). The 300b amp gave way to a pp2a3, which I still have and use, it utilizes an individual transformer for each tube, with the centertaps connected to a common cathode resistor.
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Old 1st April 2014, 03:59 AM   #19
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This is my idea.
Attached Images
File Type: jpg isolation transformer.JPG (21.4 KB, 96 views)
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Old 1st April 2014, 01:37 PM   #20
JimW is offline JimW  United States
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Wouldn't it be simpler and less expensive to get a 120vac to two 2.5 vac secondaries from an off the shelf supplier such as hammond?
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